Formation of Oxidized Organic Aerosols in Winter Haze in a Megacity of Southwest China: Implication for the Importance of Biomass Burning

As an important part of PM 2.5 , the formation mechanism of organic aerosol is still yet to be clarified due to the complexity of its composition. In this study, the composition of non‐refractory fine particles (NR‐PM 2.5 ) in an urban area in winter Chongqing was characterized by time‐of‐flight aerosol chemical speciation monitor (ToF‐ACSM). Organic aerosol (OA), accounting for 48.8% ± 14.6% of NR‐PM 2.5 , was the most abundant species. Positive matrix factorization model resolved more‐oxidized oxygenated OA (MO‐OOA) as the predominant component of OA (55.8% ± 11.1%), followed by less‐oxidized oxygenated OA (LO‐OOA) (19.0% ± 6.7%). LO‐OOA exhibited dual sensitivity to both aqueous‐phase processing (enhanced at higher O x levels) and photochemical oxidation, whereas MO‐OOA was predominantly photochemically derived. Moreover, photochemical oxidation preferentially promoted MO‐OOA formation over LO‐OOA. OA exerted a key role in forming the first haze event, which was significantly related to the enhanced production of secondary OA (SOA). Compared to the second event, the first haze exhibited significantly stronger biomass‐burning emissions, likely promoting LO‐OOA formation and its subsequent oxidation to MO‐OOA. Moreover, biomass‐burning may substantially contribute to highly oxidized OA after haze. Generally, the results of this study provide new insights into SOA formation and emphasized the need of controlling biomass burning.